This invention pertains generally to synthetic aperture radars and more particularly to a synthetic aperture radar adapted for use in a guided missile.
In a modern military operation, it is desirable to launch a missile from an aircraft toward a target on the ground with the missile having a capability of discriminating between targets. In such a weapon system, the missile must have access to targeting information and must be able to identify a specific target while in flight, thus enabling the aircraft to stay out of the dangerous area around the target.
In one type of weapon system, an aircraft and a missile are equipped with a synthetic aperture radar (SAR). A SAR mounted within an aircraft or a missile makes an image of objects within a field of view of the SAR on the underlying ground. The SAR forms a map of pixels, the pixels (pixel elements) divide objects on the ground into resolution cells. Typically, a human on the aircraft designates a specific object in the image as a target. A portion of the image about the object designated as the target becomes a reference image. After launch, the missile would fly toward the target forming its own SAR images. Data processing circuitry within the missile identifies groups of pixels representing the target. Processing on the missile would compare the reference image with the current image from the missile. The guidance system within the missile would use the results of the image comparisons to generate guidance commands to steer the missile towards the target.
In guiding a missile in this fashion, it is desirable for the SAR to initially make a map of a wide area on the ground. The wide area map helps ensure the target is somewhere in the map. A single beam from a SAR making a wide area map would typically be correspondingly broad, which can be undesirable. For example, such a SAR would be more susceptible to electronic countermeasure techniques and require lengthy real time computational processing time. One proposed solution is to use a SAR with a relatively narrow beam which is scanned over the wide area on the ground. A problem with such an approach is that the portions of the map formed at different portions of the scan need to be related to each other.
Another desirable feature of a SAR used for guiding a missile is that it makes a map with high resolution. The high resolution allows the data processing circuitry to identify relatively small targets. One known way to improve the resolution of a SAR is to increase the bandwidth i.e. sampling rate. However, components which operate at high sampling rates, especially analog to digital converters, are very expensive.
Another problem is, as the missile approaches the target, the viewing angle changes such that the SAR images formed by the missile in flight will differ from the SAR images formed at the time the missile was launched. Thus, the image processing technique must be relatively insensitive to the viewing angle.
One technique used within a missile to guide a missile toward the target is described in U.S. Pat. No. 5,027,422, issued Jun. 25, 1991, to T. J. Peregrim, et al. (which patent is assigned to the same assignee as this application and is incorporated herein by reference). In U.S. Pat. No. 5,027,422, a technique is described for providing a reference template of a designated target. The reference template is then compared to each area of a second image provided by the missile as the missile flies toward the target to find the area within the second image matching the reference template. Once the area of the second image is found to match the reference template, the area of the match indicates the location of the target in the target area image and the direction of the target in the target area image can be provided to the missile control system to adjust the line of flight of the missile. With the latter in mind, it is desirable to develop improved image processing techniques to extract surface targets from clutter and then determine a target aimpoint within the target area image.